PSI - Issue 72

P.M.M. da Silva et al. / Procedia Structural Integrity 72 (2025) 69–76

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3.4. Shear fracture tests The ENF tests were carried at room temperature at 1.5 mm/min. The a values were obtained by photographs each 5 s using the same camera. G IIC was determined by the CCM, direct beam theory (DBT), CBT and CBBM. Fig. 8 (a) shows, as an example, the evolution of P-  curves for SMC 30%. All curves show similar behavior.

2.5

450

2

300

1.5

P [N]

1

G II [N/mm]

150

0.5

0

0

0

2

4

6

8

30

35

40

45

50

55

60

δ [ mm]

a or a eq [mm] CCM DBT CBT CBBM

a)

b)

P1

P4

P7

P8

Fig. 8. P -  curves form SMC 30% (a) and R -curves by each method in an ENF specimen with SMC 30% material (b).

Fig. 8 (b) shows an R-curve for the SMC 30% highlighting G IIC calculated by the four-methods. All R -curves present a steady-state zone corresponding to the measurement of G IIC .In the case of CBBM curve, G IIC increases up to crack initiation (Azevedo et al. 2015). The average G IIC are presented in Table 6. For the SMC with 15% and 20% FVF, it was not possible to measure G IIC for the CCM, CBT, and DBT due to an early failure of the specimen. The highest percentile differences to the CBBM, considered the most robust method (de Moura et al. 2009), are 2.1% for the CCM (SMC 30%), 3.2 % for the CBT (SMC 45%) and 4.2% DBT (SMC 45%).

Table 6. G IIC values obtained by each method through ENF.

Material

CCM [N/mm]

CBT [N/mm]

DBT [N/mm]

CBBM [N/mm] 1.2374 1.4893 2.0157 2.1398

SMC 15% SMC 20% SMC 30% SMC 45%

- -

- -

- -

1.9724 2.1123

2.0464 2.2092

1.9565 2.0492

Fig. 9 shows G IIC as a function of FVF. The FVF increase promotes a 72.9% improvement in G IIC . Thus, smaller improvements are found comparatively to G IC , which leads to the conclusion that this fracture parameter is mostly dictated by the polymeric matrix. Compared to the DCB tests, in which the fibers create a bridging phenomenon that significantly improves G IC , in the ENF tests this effect cannot surge the same strengthening effect, in a geometry in which the crack grows without opening and in which the fibers are not significantly arresting crack progression.

2.5

2.0

1.5

1.0 G IIC [N/mm]

0.5

0.0

0

10

20

30

40

50

FVF [%]

Fig. 9. FVF influence on G IC (CBBM).